1. Field of the Invention
This invention relates to propellant feed systems. More specifically, the invention is a system for achieving swirl-coaxial injection of a liquid propellant (e.g., oxidizer, fuel).
2. Description of the Related Art
Effective atomization of fluids is a vital attribute of liquid rocket injectors. One commonly-used injector is the swirl-coaxial injector. Briefly, the swirl-coaxial injector forces the mixing of propellants, e.g., a liquid oxidizer and a fuel. Typically, the oxidizer is introduced under pressure at rectangular slits formed in an injector tube such that the oxidizer swirls within the tube as it moves therethrough. The fuel is introduced into the oxidizer swirl at the end of the tube so that an atomized mixture is sprayed out. Note that the fuel and oxidizer could be switched so that the fuel was swirled rather than the oxidizer.
A traditional propellant feed system for swirl-coaxial injection of a liquid propellant is illustrated schematically in FIG. 1 and is referenced generally by numeral 10. It is to be understood that FIG. 1 is a simplistic presentation of a propellant feed system in order to clearly illustrate the drawbacks associated therewith. Since these drawbacks are related to the propellant feed/injection, only the elements related to same will be shown in FIG. 1. It is further to be understood that while FIG. 1 will be described for the swirling of a liquid oxidizer and subsequent injection of a fuel, the fuel and oxidizer could be switched to allow for swirling of the fuel and subsequent injection of the oxidizer.
In the illustrated example, feed system 10 includes a liquid oxidizer cavity 12 and a fuel cavity 14 separated from one another by an interpropellant plate 16. Passing through plate 16 are a number of injector tubes 18. Each tube 18 has a number of rectangular slits 20 (only one is shown in each tube 18) formed tangentially in the walls of tube 18 in oxidizer cavity 12. The bottom of each slit 20 lies above interpropellant plate 16.
In operation, a liquid oxidizer is supplied to oxidizer cavity 12 by means of, for example, a supply line 30. Each tube 18 with slits 20 is designed to introduce a swirl of oxidizer when slits 20 are fully immersed in an oxidizer. Accordingly, oxidizer cavity 12 with interpropellant plate 16 serves as a reservoir that holds enough liquid oxidizer to immerse slits 20. When this occurs, the oxidizer is introduced into tubes 18 at the proper pressure to create the swirl flow in the tube. Although the details are omitted in FIG. 1, fuel in fuel cavity 14 is introduced into the swirl flow at the tube's exit 18A. The mixture is then sprayed from the exit 18A of each tube 18.
The problems with propellant feed system 10 typically occur at engine shut down. Specifically, liquid oxidizer flow into oxidizer cavity 12 ceases at engine shut down. At some point, the level of liquid oxidizer in oxidizer cavity 12 falls below the top of slits 20. When this occurs, the flow volume and pressure needed to introduce the swirl flow in tubes 18 falls below the steady-state design level. This negatively impacts the swirl flow needed for proper mixing with the fuel. Specifically, when there is enough liquid oxidizer remaining in oxidizer cavity 12 to partially immerse slits 20 after engine shut down, the oxidizer “dribbles” into tube 18. This “dribble volume” (as it is known) must be expelled by vaporization of the residual fluid. Still further, since the bottom of slits 20 is above interpropellant plate 16 (to allow for brazing of tubes 18 to plate 16), a pool of liquid oxidizer remains in oxidizer cavity 12. However, if hot combustion gas reverse flows through tubes 18 (as often occurs during shutdown), the remaining pool of liquid oxidizer could combust within oxidizer cavity 12 causing a detonation “pop” that can damage the injector feed system.